The present invention relates generally to a method for the recovery of lead and more particularly to a method for recovering lead from a material or ore containing lead sulfide wherein the lead sulfide containing material or ore is initially leached in a leaching vessel. The lead sulfide containing material or ore is leached in a chloride solution to which iron (III) chloride has been added as an oxidation agent, and thereafter subjected to an electrolytic treatment.
In order to obtain lead from sulfide containing materials or ores, pyrometallurgical and hydrometallurgical methods have essentially been used in the art. According to the roast-reduction method or the roast-reaction method, for example, sulfur in the form of a sulfide (lead sulfide) may be readily treated by roasting to form sulfur dioxide which is processed into sulfuric acid. After multistage refining of the resulting raw lead, fine lead is finally obtained. The treating and refining of lead sulfide ores by such methods, which ores contain in addition to lead and sulfur, inter alia, copper, zinc, antimony, arsenic, iron, cadmium as well as noble metals, produces substantial environmental pollution because the various processing steps result in the discharge of sulfur dioxide and other gaseous pollutants as well as toxic fine dusts.
In view of the environmental problems associated with pyrometallurgical processes, hydrometallurgical methods are being considered with increasing frequency. According to one such known method, for example, anodes are made to lead sulfide ones, and subjected to electrolysis. However, the poor stability of these anodes and sulfur coatings developing thereon, restrict this mode of operation to within narrow limits. Instead of the method employing preshaped anodes, methods also exist wherein lead sulfide concentrates, in suspension, are anodically dissolved. According to these suspension electrolysis methods, lead sulfide particles are intensively moved within the anode chamber of an electrolytic cell so that the particles come into frequent contact with the chemically inert anode and in a way, dissolve quasi-anodically. The basic electrolyte used is silicofluoric acid and borofluoric acid.
A disadvantage of these methods is that the anode and cathode chambers must be separated by membranes or diaphragms which are mechanically sensitive, decompose easily and exhibit a high electrical resistance. Further disadvantages of these methods are that relatively expensive, fluorine containing basic electrolytes are used and that the lead sulfide containing raw materials, including annoying ancillary components and impurities therein, are introduced into the electrolysis cell.
It is known that lead is readily soluble in solutions containing large amounts of chloride, e.g., sodium chloride, because the lead then goes into solution in the form of a chlorocomplex. Thus, a method is known wherein lead sulfide concentrates are leached at about 90.degree. C., in solutions containing about 250 g/l of sodium chloride. The sulfur, in the form of a sulfide is oxidized by copper (II) ions in accordance with the following reaction: EQU PbS+CuCl.sub.2 .fwdarw.CuS+PbCl.sub.2.
The resulting lead chloride is crystallized out by cooling, and as a fused melt it is reduced by hydrogen to form lead. In a second leaching stage, the copper sulfide containing residue is converted to copper (I) chloride and sulfur according to the following chemical reaction: EQU CuS+CuCl.sub.2 .fwdarw.2CuCl+S.degree..
In a third leaching stage, the copper (I) chloride is finally regenerated according to the following chemical reaction: EQU 2CuCl+2HCl+1/2O.sub.2 .fwdarw.2CuCl.sub.2 +H.sub.2 O,
with gaseous hydrochloric acid produced during the reduction of lead chloride and with oxygen from the air. The disadvantage of this method is that the leaching process requires relatively high temperatures in the order of 90.degree.-100.degree. C., and during the reduction of lead chloride hydrochloric acid gases are produced which present a great danger to the environment and particularly to the operating personnel.
According to another known method, lead sulfide is leached at a temperature of about 100.degree. C. in a sodium chloride solution which contains iron (III) chloride that has been added as an oxidation agent. According to the chemical reaction, the lead sulfide is leached with the hot ferric chloride-NaCl solution to obtain lead chloride and elemental sulfur as follows: EQU PbS+2FeCl.sub.3 .fwdarw.PbCl.sub.2 +S.degree.+2FeCl.sub.2
In this reaction, the iron (III) chloride is reduced to iron (II) chloride. Lead chloride crystallizes from the leach solution on cooling and thereafter is subjected to fused salt electrolysis, wherein the lead is deposited cathodically and gaseous chlorine develops anodically which serves to reoxidize the iron (II) chloride. This method has the same drawbacks as the preceding method.
In a further known hydrometallurgical method, an electrolysis cell is used which is subdivided into an anode chamber and a cathode chamber by a permselective membrane which permits anions to pass therethrough. In this method, lead sulfide, in a sodium chloride solution containing iron chloride, is subjected to a suspension electrolysis at about 70.degree. C. in the anode chamber, whereby the sulfur in the form of a sulfide (lead sulfide) is oxidized to elemental sulfur and lead chloride is produced with can be crystallized out.
The lead chloride is purified by recrystallization and, after renewed dissolving, is brought into the cathode chamber of the electrolysis cell wherein lead is deposited. Since the cathode chamber and the anode chamber are separated from one another by the membrane which permits anions to pass therethrough, the chloride ions can move over to the anolyte. In this mode of operation, toxic gaseous reaction products are avoided, but the crystallization and redissolving of the lead chloride, for purposes of purification, are rather complicated. The greater problem encountered, however, is that the electrolytic cell is divided into chambers by the permselective membrane. Since this membrane is mechanically sensitive, it clogs easily causing a considerable voltage drop and thus, it presents significant disadvantages when used in the large-scale production of lead.
It all of the prior art methods used for the hydrometallurgical recovery of lead from sulfide containing raw materials, the lead first forms lead chloride which is separated from the liquor by crystallization. The reduction of the lead chloride takes place either in a fused melt, whereby hydrochloric acid or chlorine are released or in an aqueous solution in an electrolytic cell employing a permselective membrane. Thus, the known prior art methods either result in the formation of toxic gases deleterious to the environment or the conditions under which the apparatus are employed prove to be difficult so that these methods can be used only with great restrictions. A need therefore exists for a method to recover lead from lead sulfide containing materials, including ores and concentrates, that avoids the problems previously encountered in prior art processes.